Temperature measurement of a power semiconductor switching element

ABSTRACT

A device for determining a temperature of a semiconductor power switch with a built-in temperature-dependent gate resistor may include a non-inverting amplifier circuit comprising an operational amplifier and a feedback resistor. Inverting input of the operational amplifier may be connected to the semiconductor power switch such that a gain of the non-inverting amplifier circuit in a predefined frequency range of an input signal depends on the built-in temperature-dependent gate resistor and the feedback resistor and is a measure of the temperature of the semiconductor power switch. The feedback resistor may be disposed between a negative input and an output of the operational amplifier.

The present invention relates to a device for determining a temperatureof a semiconductor power switching element having the features of thepreamble of claim 1 and to methods for measuring a temperature of asemiconductor power switching element having the features of thepreambles of claims 9 and 10.

Power elements have a significant power loss which causes the chips toheat up to a temperature which may be considerably higher than theambient temperature. The junction temperature is the most importantvariable of a power switch to be limited. The behavior of powerelements, such as power switches, is negatively affected by excessivelyhigh temperatures. If a maximum permissible temperature is exceeded,there is a threat of a thermal event in the relevant component and thecomponent no longer functions correctly or completely fails.

The published patent application DE 10 2014 204 648 A1 discloses amethod for determining a temperature of an insulated gate bipolartransistor (IGBT). A driver for providing a first control voltage at thegate of the IGBT and a control voltage generator for providing a secondcontrol voltage at the gate of the IGBT are provided, wherein the driverand the control voltage generator are configured to be operated in analternating manner, with the result that only one of the controlvoltages is ever present at the gate. The second control voltagecomprises a DC voltage component and a superposed AC voltage componentsuch that the IGBT is kept in blocking operation. A parasiticcapacitance conducts the AC voltage, even though the IGBT remains in theblocking state. The gate current flows through the temperature-dependentinput resistor, with the result that, in the case of a predeterminedcontrol voltage and a predetermined amplitude of the AC voltage, thegate current is a measure of the temperature of the IGBT, as a result ofwhich the temperature of the IGBT can be determined.

DE 10 2012 102 788 A1 discloses a measurement of the junctiontemperature of a MOSFET, wherein a diode is provided on board, thecathode of which is internally connected to the source of the MOSFET, asa result of which the number of connections which are routed out and thechip area of the component can be reduced. The diode is directly coupledto the depletion layer of the MOSFET, thus making it possible todirectly measure the junction temperature of the MOSFET. A current whichis used to operate the diode in the forward direction is used todetermine the junction temperature. The current flow through the diodegenerates a forward voltage across the diode that istemperature-dependent and current-dependent. This voltage can bemeasured between the anode of the diode and the source connection of theMOSFET.

The known temperature measurements are largely dependent on the currentintensity, which results in inaccuracies in the determination of thetemperature, in particular when using the semiconductor power switchingelements in engine control units.

The object of the present invention is to specify a device fordetermining a temperature of a semiconductor power switching element,which device determines the temperature in a particularly accurate andreliable manner independently of the current intensity of the switchedcurrent.

This object is achieved by a device for determining a temperature of asemiconductor power switching element having the features of claim 1 andmethods for determining a temperature of a semiconductor power switchingelement having the features of claims 9 and 10.

Accordingly, a device for determining a temperature of a semiconductorpower switching element having the semiconductor power switch with abuilt-in temperature-dependent gate resistor is provided, wherein thedevice has a non-inverting amplifier circuit comprising an operationalamplifier and a feedback resistor, wherein the operational amplifier isconnected to the semiconductor power switch in such a manner that thegain of the non-inverting amplifier circuit in a predefined frequencyrange of an input signal depends on the built-in temperature-dependentgate resistor and the feedback resistor and is a measure of thetemperature of the semiconductor power switching element.

The measurement of the temperature using the most precisely knowntemperature dependence of the gate resistor is particularly reliable andindependent of the current intensity. The calculated temperature is agood measure of the temperature of the depletion layer of thesemiconductor power switch. In the predefined frequency range, the gainexhibits a dependence on the temperature of the gate resistor. It ispreferred for the input impedance of the semiconductor power switch tobe formed only or substantially by the gate resistor. This situation isparticularly advantageous since the gain then exhibits the greatestdependence on the temperature of the gate resistor and a change in thetemperature can be measured easily and accurately.

The feedback resistor is preferably arranged between the negative inputand the output of the operational amplifier and/or thetemperature-dependent gate resistor is arranged between the negativeinput of the operational amplifier and a reference voltage (normallyground) of the device.

The predefined frequency range is preferably above a frequency of a poleof a transfer function of an ideal non-inverting amplifier circuit.

The semiconductor power switching element is preferably a power MOSFETor an IGBT.

An electromechanical motor vehicle steering system having a multiphasepermanently excited electric motor which can be controlled via anelectronic control unit, wherein the electronic control unit has amultiplicity of semiconductor power switching elements which are part ofan inverter, is also provided, wherein each of the semiconductor powerswitching elements has an above-described device for determining atemperature of the respective semiconductor power switching element. Thesystem can also comprise the operation of switching semiconductor relaysfor each phase and may comprise means for determining an above-describedtemperature, which means have the respective semiconductor powerswitching element. In one preferred embodiment, the electric motor isthree-phase and has two semiconductor power switching elements for eachphase in a half-bridge circuit, which semiconductor power switchingelements can be controlled by means of pulse width modulation. Othersafety relays may or may not be included.

A method for measuring a temperature of a semiconductor power switchingelement having a built-in temperature-dependent gate resistor is alsoprovided, wherein a non-inverting amplifier circuit having anoperational amplifier and a feedback resistor, which is arranged betweenthe negative input and the output of the operational amplifier, whereinthe temperature-dependent gate resistor is arranged between the negativeinput of the operational amplifier and a negative input or a referenceinput of the overall circuit, and wherein the method has the followingsteps of:

-   -   operating the overall circuit with an input signal having a        frequency which is in a predefined frequency range, with the        result that the built-in temperature-dependent gate resistor        forms a substantial part of the input impedance of the        semiconductor power switch,    -   measuring the gain of the non-inverting amplifier circuit,    -   calculating the resistance of the built-in temperature-dependent        gate resistor by means of the measured gain and determining the        temperature of the semiconductor power switching element.

Another possibility is to calibrate the system since the output signalhas a precisely defined dependence on the value of the gate resistor:

-   -   operating the overall circuit with an input signal having a        frequency component which is in a predetermined frequency range,        with the result that the built-in temperature-dependent gate        resistor forms a substantial part of the input impedance of the        semiconductor power switch,    -   measuring the output signal from the non-inverting amplifier for        at least two temperature values,    -   calculating the temperature dependence of the output signal on        the basis of a-priori knowledge,    -   determining the temperature of the semiconductor power switch on        the basis of the actual output signal and the predetermined        temperature dependence.

The two methods allow the temperature of the semiconductor powerswitching element to be determined in a particularly accurate andreliable manner independently of the current intensity of the switchedcurrent.

The determined temperature is a measure of the junction temperature ofthe semiconductor power switching element.

The semiconductor power switching element is preferably a power MOSFETor an IGBT.

The predefined frequency range is above a frequency of a pole of atransfer function of an ideal non-inverting amplifier circuit.

Two preferred embodiments of the invention are explained in more detailbelow on the basis of the drawings. Identical or functionally identicalcomponents are provided in this case with the same reference signsthroughout the figures, in which:

FIG. 1: shows a circuit diagram of a device for determining atemperature of a power switching element having a power switch and anon-inverting amplifier circuit,

FIG. 2: shows a circuit diagram of a device for determining atemperature of a power MOSFET, and

FIG. 3: shows a graph with a transfer function of an ideal and a realnon-inverting amplifier circuit.

FIG. 1 illustrates a circuit having a power MOSFET 5, which acts as asemiconductor power switching element 1, with a built-in gate resistor3.

The semiconductor power switch 1 has a parallel circuit (notillustrated) of a multiplicity of individual semiconductor switchesarranged on a common chip. A significant advantage of the semiconductorpower switches 1 is the high possible switching frequency which isadvantageous, for example, for pulse width modulation in an motorcontroller. The power MOSFET 5 has a built-in gate resistor 3 which isprovided for the purpose of balancing the current distribution betweenthe individual semiconductor switches on a chip in order to avoidparasitic oscillations and to reduce the Q factor of a possible RLCseries circuit at the input. The built-in gate resistor 3 is part of aninput impedance of the power MOSFET 5. The built-in gate resistor 3 hasa known temperature dependence which is a measure of the temperature ofthe depletion layer of the semiconductor power switching element 1. Thetemperature-dependent change in the resistance of the gate resistor 3 isdetected by means of a non-inverting amplifier circuit 200 comprising anoperational amplifier 2, a feedback resistor 4 and the built-in gateresistor 3. The semiconductor power switch 1, with its input impedance,is arranged between the negative input of the operational amplifier 2and a setpoint input of the complete circuit V_(IN−).

The voltage V_(IN+) to be amplified of the setpoint input of theoperational amplifier 2 is applied to the non-inverting, positive inputof the operational amplifier 2. A fraction of the output voltageV_(OUT+) from the operational amplifier 2 is fed back to the inverting,negative input as negative feedback by means of voltage division usingtwo resistors. The feedback resistor 4 is arranged between the negativeinput of the operational amplifier and the output.

The input impedance of the semiconductor power switch 1 can be modeledusing an RC series circuit (see FIG. 2) having an input capacitor and aninput resistor. In the case of a particular input signal, the seriescapacitance of the RC series circuit can be considered to be a shortcircuit, with the result that the input impedance is formed only by thebuilt-in gate resistor 3. The gain of the non-inverting amplifiercircuit 200 depends on a frequency f. In the case of frequencies above acut-off frequency f_(P), the input impedance is formed by the built-ingate resistor 3. The impedance of the capacitor 10 is reduced and can beconsidered to be a shortcut. This effect can be observed to anincreasing extent with increasing frequency.

The cut-off frequency f_(P) is calculated using the following formula:

$f_{P} = \frac{1}{2\pi C_{GS}R_{G}}$

where C_(is) is the input capacitor 10, R_(G) is the gate resistor 3(see FIG. 2).

In this case, the output of the operational amplifier 2 must adjust aratio of the feedback resistor 4 and the gate resistor 3 in order tocontrol the voltage at the negative input to that of the positive inputV_(IN+). The terminal gain of the operational amplifier between theinput and output terminals is provided solely by the feedback resistor 4and the gate resistor 3.

On account of the temperature dependence of the gate resistor 3, thegain of the non-inverting amplifier circuit 200 is a measure of thetemperature of the depletion layer of the semiconductor power switch.

FIG. 2 shows a simplified model of a power MOSFET 5 which has a drainconnection 6, a source connection 7 and a gate connection 8. The powerMOSFET 5 is used in the above-described circuit for measuring thetemperature of the depletion layer. In addition to the built-in gateresistor 3, the RC series circuit 9 for modeling the input impedances atthe source connection 7 and drain connection 6 is also illustrated. Aninput capacitor 10 of the RC series circuit 9 symbolizes the capacitanceof the gate, which is an intrinsic property of any MOSFET. The resistors3 connected in series are installed in order to ensure a uniformdistribution of voltage over the respective MOSFETs on the chip.Furthermore, a common input resistor 3 is provided and is intended toprevent the presence of high-frequency oscillations. The non-invertingamplifier circuit 200 operates as an amplifier with a gain factor of 1for DC voltage signals. In this case, the capacitor 10 of the powerMOSFET is treated as idling and the remaining circuit represents a gainof 1. The circuit begins to operate as an amplifier with an increase inthe frequency of the voltage signal.

FIG. 3 illustrates the transfer function of the non-inverting amplifiercircuit 14. In this case, the gain of the non-inverting amplifiercircuit is plotted against the frequency.

The asymptotically approximated transfer function of an idealnon-inverting amplifier circuit 12 is illustrated as a dashed line.

The transfer function of the non-inverting amplifier circuit 14comprises a zero at f_(Z) and a pole at f_(P), wherein the frequency ofthe zero f_(Z) is always less than the frequency of the pole f_(P). Thetransfer function therefore exhibits the characteristics of a high-passfilter. If a true operational amplifier is used, the characteristicsexhibit an additional pole and passband-like filtering. The horizontalarrow 13 indicates the frequency range in which the signal is amplifiedby the filter. The gain increases in the range between the zero and thepole, but the capacitor of the RC series circuit still exhibits enoughimpedance to superpose the slight temperature-related changes in thegate resistance, with the result that they are not visible in the gain.For frequencies above the pole, the impedance of the capacitor hasslightly more influence on the input impedance and the capacitor behavesas if it were short-circuited. The gain is dependent on thetemperature-dependent resistance. The vertical arrow 15 indicates thetemperature-related fluctuation in the measured gain.

For frequencies above the pole f>f_(P), the gain v is only dependent onthe gate resistor R_(G)(T) and the feedback resistor RF, like in anormal non-inverting amplifier:

${v(T)} = {1 + {\frac{R_{F}}{R_{G}(T)}.}}$

The invention is not limited to MOSFETs. It is also possible to useother semiconductor power switching elements which have atemperature-dependent resistor at a control input.

Semiconductor power switching elements are used, for example, in thephase winding of an electric motor of a steering system of a motorvehicle, preferably in the form of half-bridges, in particular a triplehalf-bridge for controlling a three-phase motor. The choice of asuitable semiconductor component results from the desired switchingbehavior. Power MOSFETs are preferably used as semiconductor components,but other components, for example IGBTs, can also be used. Thetemperature information determined using the apparatus according to theinvention can be used, for example, to protect the MOSFETs from thermaloverloading. If a critical junction temperature is reached, for example,steering assistance of an electromechanical steering system of a motorvehicle can be reduced and the power loss can therefore be reduced.

1.-13. (canceled)
 14. A device for determining a temperature of asemiconductor power switch with a built-in temperature-dependent gateresistor, the device comprising a non-inverting amplifier circuitincluding an operational amplifier and a feedback resistor, wherein aninverting input of the operational amplifier is connected to thesemiconductor power switch such that a gain of the non-invertingamplifier circuit in a predefined frequency range of an input signaldepends on the built-in temperature-dependent gate resistor and thefeedback resistor and is a measure of the temperature of thesemiconductor power switch.
 15. The device of claim 14 wherein thefeedback resistor is disposed between a negative input and an output ofthe operational amplifier.
 16. The device of claim 14 comprising aninput, wherein the built-in temperature-dependent gate resistor isdisposed between a negative input of the operational amplifier and theinput of the device.
 17. The device of claim 14 wherein in thepredefined frequency range of the input signal the built-intemperature-dependent gate resistor forms a substantial part of inputimpedance of the semiconductor power switch.
 18. The device of claim 14wherein in the predefined frequency range of the input signal thebuilt-in temperature-dependent gate resistor forms a majority of inputimpedance of the semiconductor power switch.
 19. The device of claim 14wherein the predefined frequency range is above a frequency of a pole ofa transfer function of an ideal non-inverting amplifier circuit.
 20. Thedevice of claim 14 wherein the semiconductor power switch is a powerMOSFET or an IGBT.
 21. An electromechanical steering system of a motorvehicle having a multiphase permanently excited electric motor that iscontrollable via an electronic control unit, wherein the electroniccontrol unit includes semiconductor power switches that are part of aninverter and/or disposed as a semiconductor relay in each phase, whereineach of the semiconductor power switches includes the device of claim 14for determining a temperature of the respective semiconductor powerswitch.
 22. The electromechanical steering system of claim 21 whereinthe multiphase permanently excited electric motor is three-phase andincludes two of the semiconductor power switches for each phase in ahalf-bridge circuit, wherein the semiconductor power switches arecontrollable by way of pulse width modulation.
 23. A method formeasuring a temperature of a semiconductor power switch having abuilt-in temperature-dependent gate resistor via a circuit comprising anon-inverting amplifier circuit having an operational amplifier and afeedback resistor that is disposed between a negative input and anoutput of the operational amplifier, wherein the built-intemperature-dependent gate resistor is disposed between the negativeinput of the operational amplifier and an input of the circuit, themethod comprising: operating the circuit with an input signal having afrequency that is in a predefined frequency range such that the built-intemperature-dependent gate resistor forms a substantial part of inputimpedance of the semiconductor power switch; measuring a gain of thenon-inverting amplifier circuit; and calculating a resistance of thebuilt-in temperature-dependent gate resistor by way of the measured gainand determining the temperature of the semiconductor power switch. 24.The method of claim 23 wherein the determined temperature is a measureof a junction temperature of the semiconductor power switch.
 25. Themethod of claim 23 wherein the semiconductor power switch is a powerMOSFET or an IGBT.
 26. The method of claim 23 wherein the predefinedfrequency range is above a frequency of a pole of a transfer function ofan ideal non-inverting amplifier circuit.
 27. A method for measuring atemperature of a semiconductor power switch having a built-intemperature-dependent gate resistor via a circuit comprising anon-inverting amplifier circuit having an operational amplifier and afeedback resistor that is disposed between a negative input and anoutput of the operational amplifier, wherein the built-intemperature-dependent gate resistor is disposed between the negativeinput of the operational amplifier and an input of the circuit, themethod comprising: operating the circuit with an input signal having apredefined frequency that is in a predetermined frequency range suchthat the built-in temperature-dependent gate resistor forms an integralpart of input impedance of the semiconductor power switch; measuring anoutput signal from the non-inverting amplifier circuit for at least twotemperature settings; calculating a temperature dependence of the outputsignal; and determining the temperature of the semiconductor powerswitching element based on the output signal and the calculatedtemperature dependence.
 28. The method of claim 27 wherein thedetermined temperature is a measure of a junction temperature of thesemiconductor power switch.
 29. The method of claim 27 wherein thesemiconductor power switch is a power MOSFET or an IGBT.
 30. The methodof claim 27 wherein the predetermined frequency range is above afrequency of a pole of a transfer function of an ideal non-invertingamplifier circuit.